Optical head

ABSTRACT

In an optical head, a laser beam emitted from a laser unit is collimated by a collimater lens and is transmitted through a polarized beam splitter and λ/4 plate. The collimated laser beam is converged onto a recording layer of an optical disk by an objective lens. A beam spot corresponding to the beam waist of the converged laser beam is formed on the recording layer in a just-in-focusing state and a beam spot larger than the beam spot corresponding to the beam waist is formed on the recording layer in a defocusing state. The laser beam reflected from the recording layer passes through the objective lens and is λ/4 plate and refracted by the polarized beam splitter. The polarized beam splitter has first and second beams emerging surfaces the first beam emerging surface being arranged perpendicularly to the optical axis and second beam emerging surface being arranged at a predetermined angle to the first beam emerging surface. Accordingly, the laser beam is separated into first and second laser beam components at the first and second beam emerging surfaces by the polarized beam splitter, which are directed in different directions. The first laser beam component is incident on a photodetector for detecting the focusing state of the objective lens.

This is a division of application Ser. No. 682,922, filed Dec. 18, 1984now U.S. Pat. No. 4,654,519.

BACKGROUND OF THE INVENTION

The present invention relates to an optical head for focusing an opticalbeam for recording/reproducing information or recording, reproducing anderasing information on an information recording medium such as anoptical disk and, more particularly, to a focus detecting system fordetecting the displacement of the focus of an optical head.

Recently, various systems for recording information on an informationrecording medium, such as an optical disk reproducing or erasing therecorded information, have been developed and proposed. In thesesystems, a beam spot corresponding to a beam waist of an optical beamconverged by an objective lens at recording, reproducing and erasingtime is required to be formed on an optical disk. Thus, an optical headhas a focus detecting system for detecting the focused state of thelens. For example, a focus detecting system which is called a so-called"knife edge method" was proposed as disclosed in U.S. patent applicationSer. No. 399,873 filed on July 19, 1982 by the same assignee as thisapplication, and an EPC application No. 82106508.3, filed on July 19,1982 as a focus detecting system. In this focus detecting system whichis called the knife edge method, a light shielding plate which shieldspart of an optical beam reflected from a light reflecting surface andpermits passage of other components is arranged between the lightreflecting surface of the optical disk, i.e., information recordingsurface and a photo-detector for detecting the focus. The other beamcomponent is directed between a pair of photo sensitive regions of aphoto-detector at the focusing time, and the level of the detectedsignals from a pair of photo sensitive regions is maintainedsubstantially and equally. The other beam component is directed to oneof the pair of photo sensitive regions of the photo-detector atnonfocusing time, and the level of the detected signals from the pair ofphoto sensitive regions is unbalanced. Therefore, whether the objectivelens is focused or not can be judged according to the detected signalsfrom the pair of photo sensitive regions.

In the focus detecting system, such as the so-called "knife edgemethod," it is required to accurately dispose the light shielding platein an optical passage of the light beam, and it is necessary to paycareful attention in alignment at assembling time, and there arises aproblem that the assembling work becomes complicated. Further, a spacefor disposing the light shielding plate is required, the space should besufficiently large to simplify the assembling work, and the systemitself cannot be accordingly reduced compact in size.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an optical head whichcan be readily assembled in a compact structure.

According to one aspect of the present invention, there is provided anoptical head for focusing a light beam onto an information recordinglayer comprising a light source for generating a light beam,

an objective lens for converging the light beam toward the informationlayer, transferring the light beam reflected from the informationrecording layer so that a beam spot corresponding to the beam waist ofthe light beam is formed on the recording layer in the just-in-focusingstate, and a beam spot larger than the beam spot corresponding to thebeam waist of the light beam is formed on the recording layer in adefocusing state,

a first refractive member having first and second light beam emergingsurfaces for separting the light beam transferred from the objectivelens into first and second light beam components directed toward thedifferent direction,

means for converting the first and second light beam components, and

first photo-detecting means having first and second photo sensitiveregions so that the first converged light beam component is incidentthereon.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing an optical head of a first embodimentaccording to the present invention;

FIGS. 2A, 2B, 2C are plan views respectively showing beam spots formedon a photo-detector for focusing and detecting in just-in-focusing anddefocusing state the optical head shown in FIG. 1;

FIGS. 3A, 3B, 3C are plan views respectively showing refractive patternsin the beam spot formed on the photo-detector for detecting informationand for detecting track in the just-in-focusing state in the opticalhead shown in FIG. 1; and

FIG. 4 is a schematic view showing an optical head of a secondembodiment according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematic view showing an optical head of a first embodimentaccording to the present invention. In FIG. 1, reference numeral 21designates a semiconductor laser device. A laser beam L generated fromthe laser device 21 is converted into parallel luminous flux through acollimater lens 22, and directed toward a polarized beam splitter 24.The parallel laser beam L passed through the splitter 24 is incidentthrough a λ/4 plate 25 to an objective lens 26, thereby focusing thebeam L toward the light reflecting layer or the recording layer 28 of aninformation recording medium, i.e., an optical disk 27. The optical disk27 has a tracking guide (not shown) which extends along a direction Y0perpendicular to the optical axis. The lens 26 is supported and can bemoved along the optical axis by a voice coil 29. When the lens 26 isdisposed at a predetermined position, the beam waist of the focusinglaser beam L emitted from the lens 26, is projected to the lightreflecting layer or the recording layer 28, and the beam waist spot isformed on the surface of the light reflecting layer or the recordinglayer 28. In this state, the lens 26 is held in the focused state, andthe information can be written, read out or erased. When the informationis written, the state of the region on a tracking guide on the lightreflecting layer or the recording layer 28 is varied, e.g., a pit isformed by the photointensity-modulated laser beam L. When theinformation is read out, the laser beam L having a predetermined lightintensity is photointensity-modulated and reflected by the pit formed ona tracking guide. When the information is erased, the laser beam Lhaving a predetermined light intensity is directed to a tracking guide,and the region is returned to the original state. A divergent laser beamL reflected from the light reflecting layer or the recording layer 28 ofthe information recording medium 27 is converted to a parallel luminousflux through the lens 26 at the focusing time, and returned againthrough the λ/4 plate 25 to the splitter 24. When the beam Lreciprocates to the λ/4 plate 25, the beam L is rotated at 90° at thepolarized plane as compared with the case that the beam L is reflectedby the splitter 24, the beam L thus rotated at the polarized plane at90° does not pass the splitter 24, but is reflected on the splitter 24.

The splitter 24 is formed by bonding a pair of prisms 23-1 and 23-2through a polarized surface 24-1 as shown in FIG. 1. This splitter 24 isdifferent from the ordinary polarized beam splitter, but the prism 23-2for passing the laser beam reflected from the light reflecting surface28 has a function for separating the reflected laser beam into twocomponents. More specifically, the laser beam reflected from thepolarized surface 24-1 is passed along the optical axis in the prism23-2 and emitted out of the prism 23-2 through the two light emittingsurfaces 30-1 and 30-2 of the prism 23-2. One light emitting surface30-1 crosses perpendicularly to the optical axis, and the other lightemitting surface is arranged at a predetermined angle to the one lightemitting surface 30-1. Both the light emitting surfaces 30-1, 30-2 arearranged along a direction Y1 which is parallel with an image of thedirection Y0 formed on the one light emitting surface 30-1 by theoptical elements 26, 25, 30 when the direction Y0 into which thetracking guide is extended is transferred by the optical elements 25,26, 30. Thus, one component L1 of the laser beam passed through onelight emitting surface 30-1 rectilinearly propagates, while the othercomponent L2 of the laser beam passed through the other light emittingsurface 30-1 is refracted on the surface, and directed toward thedirection different from the one component L1. The laser beam componentsL1 and L2 are all converged by a projection lens 31, one laser beamcomponent L1 is directed toward a photo-detector 32 for detecting thefocusing state, and the other laser beam component L2 is directed towarda photo-detector 33 for detecting information and track. In an opticalsystem shown in FIG. 1, the other light emitting surface is determinedso that the optical paths of the laser beam components L1 and L2intersect.

The photo-detector 32 for detecting the focusing state is arranged on animage forming plane on which the image of the beam spot corresponding tothe beam waist is formed by an optical system between the photo-detector32 and the light reflecting layer 28 when the objective lens 26 ismaintained in the just in focusing state and the beam spot correspondingto the beam waist is formed on the recording layer 28. For example, whenthe parallel laser beam is transmitted from the lens 26 to the lens 31in the just-in-focusing state, the photo-detector 32 is disposed on therear focusing plane of the lens 31. On the contrary, the photo-detector33 for detecting the information and the track is disposed in the spaceexcept the image forming plane, i.e., on a far field plane.

The photo-detector 32 for detecting the focusing state has first andsecond photo sensitive regions 32-1 and 32-2 divided by photoinsensitive regions 36-3 as shown in FIG. 2A. The laser beam componentsL1 are directed toward the photo insensitive region 32-3 of thephoto-detector 32 at the focusing time, and the minimum beam spot S1 isformed as shown in FIG. 2A. The spot S1 is partly formed on the photosensitive regions 32-1 and 32-2, and these areas are formed equally toeach other. Therefore, a focusing signal of zero level is generated froma focusing signal generator 34 for generating a difference signalcorresponding to the difference of the electric signals from the photosensitive regions 32-1 and 32-2 to a voice coil driver 36, with theresult that the objective lens remains at its position. When the lens 26approaches to or separates from the light reflecting surface 28 from theposition in the just-in-focusing state, the laser beam components L1 aredirected toward either one of the photo sensitive regions 32-1 and 32-2of the photo-detector 32, and beam spots S2 and S3 larger than theminimum beam spot S1 are formed as shown in FIG. 2B or FIG. 2C. Thespots S2 and S3 are almost formed on any of the photo sensitive regions32-1 and 32-2. Accordingly, a focusing signal of plus or minus level isgenerated from the focus signal generator 34 for generating thedifference signal corresponding to the difference of the electricsignals from the photo sensitive regions 32-1 and 32-2 to the voice coildriver 36. As a result, the objective lens is moved to approach thelight reflecting surface 28 or to separate from the surface by the voicecoil driver 29, in order to move to the focusing state.

The photo-detector 33 disposed on the far field plane for detecting theinformation and the track has first and second photo sensitive regions33-1 and 33-2 divided by the photo insensitive region 33-3 as shown inFIG. 3A. The components L2 is converged by the lens 31 toward thephoto-detector 33 for detecting the information and the track, and abeam spot S4 larger than the minimum beam spot S1 is formed even at thejust-in-focusing state. A signal processor 40 connected to the first andsecond photo sensitive regions 33-1 and 33-2 of the photo-detector 33adds the electric signals from the first and second photo sensitiveregions 33-1 and 33-2 of the photo-detector 33 in the just-in-focusingstate and converts into an information signal corresponding to theinformation recorded on the recording layer 28. A diffracted image ofthe tracking guide is formed as a dark area in the beam spot S4, but thediffracted image D0 of the tracking guide is formed over two photosensitive regions 33-1 and 33-2 substantially in the same area while thelaser beam directed from the lens 26 toward the light reflecting layer28 is correctly tracing the tracking guide. Consequently, a trackingsignal of zero level is generated from the tracking signal generator 37for generating the difference signal corresponding to the difference ofthe electric signals from the photo sensitive regions 33-1 and 33-2 to alinear actuator 38, with the result that the signal remains at theposition. When the laser beam directed from the lens 26 toward the lightreflecting surface 28 does not correctly trace the tracking guide, thecomponents L2 form a beam spot S4 on the photo sensitive regions 33-1and 33-2 of the photo-detector 33, and diffracted image D0 is formed onany of the photo sensitive regions 33-1 and 33-2 as shown in FIG. 3C.Therefore, a tracking signal of plus or minus level is generated fromthe tracking signal generator 37 for generating the difference signalcorresponding to the difference of the electric signals from the photosensitive regions 33-1 and 33-2 to the linear actuator 38, with theresult that an optical head is moved by the actuator 38 in a directionparallel to the light reflecting surface 28, and the laser beam iscorrectly directed to the tracking guide.

In the optical head shown in FIG. 1, the other light emitting surface isdetermined to intersect the optical paths of the laser beam componentsL1 and L2. However, the light emitting surface may be determined asshown in FIG. 4 so that the optical paths of the L1 and L2 do notintersect, but extend in different directions.

In the optical head shown in FIG. 1, the splitter 24 is formed bybonding a pair of prisms 23-1 and 23-2 through a polarized surface 24-1.However, the prism 23-2 may be separated and disposed in the splitter24, and an ordinary polarized beam splitter is used as the splitter 24,and a prism may be newly provided as a member for separating the laserbeam into two components. Further, a half mirror (not shown) may be usedas a beam splitter instead of the polarized beam splitter.

According to the present invention as described above, the optical headof the invention does not necessitate the use of the knife edge, therebyreadily assembling the head and forming the head itself in a compactstructure.

What is claimed is:
 1. An optical head for focusing a light beam onto aninformation recording layer comprising:a light source for generating alight beam; an objective lens for converging the light beam toward theinformation recording layer and for transferring the light beamreflected from the information recording layer, so that a beam spotcorresponding to the beam waist of the light beam is formed on theinformation recording layer in the just-in-focusing state and a beamspot larger than the beam spot corresponding to the beam waist of thelight beam is formed on the information recording layer in a defocusingstate; a first refractive member having first and second light beamemerging surfaces for separating the light beam received from theobjective lens into first and second light beam components, the firstand second light beam components being directed in different directions;means for converging the first and second light beam components; andfirst photo-detecting means having first and second photo sensitiveregions to which the converged first light beam component received fromsaid converging means is directed.
 2. The optical head according toclaim 1, further comprising:an optical system arranged between the lightsource and the objective lens, for passing a light beam directed fromthe light source to the objective lens, for reflecting the light beamreceived from the objective lens to the photo-detecting means, saidoptical system including a λ/4 plate and a polarized beam splitter. 3.The optical head according to claim 2, wherein the polarized beamsplitter comprises at least two splitter refractive members bonded toeach other.
 4. The optical head according to claim 1, furthercomprising:second photo-detecting means having first and second photosensitive regions so that the converged second light beam componentreceived from said converging means is incident thereon.
 5. The opticalhead according to claim 1, wherein the first and second photo sensitiveregions of said first photo-detecting means are disposed substantiallyon an image forming plane on which the image of a beam spot on theinformation recording layer is formed by an optical system arrangedbetween the information recording layer and the photo-detecting means inthe just-in-focusing state.
 6. The optical head according to claim 1,further comprising:means connected to the first and second photosensitive regions of said first photo-detecting means for generating afocusing signal meaning the just-in-focusing state of the objective lensby the electric signals from the first and second photo sensitiveregions; and means for moving the objective lens along the optical axisthereof in response to the focusing signal.
 7. The optical headaccording to claim 1, wherein said light reflecting layer has a trackingguide, further comprising:second photo-detecting means having first andthe photo sensitive regions so that second converged second light beamcomponent is incident thereon; means connected to the first and secondphoto sensitive regions of said second photo-detecting means forgenerating a tracking signal for determining whether or not the lightbeam correctly traces the tracking guide, from the electric signals fromthe first and second photo sensitive regions; and means for moving theobjective lens in response to the tracking signal in a directionperpendicular to the optical axis thereof.
 8. The optical head accordingto claim 1, wherein the information recording layer has a tracking guideand the first and second light beam emerging surfaces are arranged alonga direction parallel to an image of the tracking guide formed on thefirst light beam emerging surface by the objective lens and the firstrefractive member.